1. Field of the Invention
The present invention relates to a method and apparatus for producing compensated baseband signal components generated by a moving radar antenna.
2. Description of Related Art
In a radar system, the signal received by the radar antenna is converted into a baseband signal and evaluated as a function of the Doppler frequency f.sub.D.
FIG. 1A illustrates a representative frequency spectrum for a non-moving (stationary) radar antenna as a function of the Doppler frequency f.sub.D (abscissa). The signal components originating from stationary targets, also called (fixed target) clutter CL, are located at the Doppler frequency f.sub.D =0. Signal components originating from targets Z1 and Z2, each moving at different velocities, are located at Doppler frequencies f.sub.21 and f.sub.22 respectively.
If a radar antenna is positioned on a moving carrier, for example, an aircraft, the representative frequency spectrum shown in FIG. 1B is produced which is a function of the velocity of the carrier. This spectrum differs from the spectrum of FIG. 1A by a shift of the frequency component f.sub.CL (clutter frequency) and target Doppler frequencies f.sub.21 and f.sub.22 as a function of the velocity of the moving carrier.
For evaluation of a spectrum such as that in FIG. 1B, it is advisable to initially shift the spectrum so that clutter frequency f.sub.CL is again positioned at the Doppler frequency f.sub.D =0 (FIG. 1C).
Such a frequency shift is possible with a conventional circuit arrangement shown in FIG. 2. The signal received by the moving radar antenna contains a frequency component f.sub.E =f.sub.ZF +f.sub.sig +f.sub.CL in the intermediate frequency (IF) range, where f.sub.ZF is the intermediate frequency, f.sub.sig is the frequency component originating from moving targets (the signal component to be evaluated) and f.sub.CL is the clutter frequency. Such an input spectrum is shown schematically in FIG. 2A for a moving target. With the aid of a quadrature demodulator, a signal is converted into a baseband signal containing signal components I and Q. The quadrature demodulator includes mixers MI and MQ and a detunable frequency oscillator Coho, also called a coherent oscillator. The oscillator signal generated by oscillator Coho is applied directly to mixer MQ. Mixer MI has a phase shifter PH connected to its input to shift the oscillator signal in phase by .pi./2 (90.degree.). Oscillator Coho is detuned by an adjustment value signal ST corresponding to clutter frequency f.sub.CL and determined in an evaluation unit AE. Evaluation unit AE considers translatory velocity v of the carrier (and antenna), and azimuth setting Az and elevation setting El of the radar antenna with respect to the bearing of the carrier, according to the following formula: EQU f.sub.CL .apprxeq.2v/.lambda..multidot.cos Az .multidot.cos El
where, .lambda. is the wavelength of the transmitted radar signal.
Detuning of oscillator Coho must always be changed corresponding to angles Az and El, which are determined, for example, by angle sensors of the antenna, and velocity v, so clutter frequency f.sub.CL can be estimated. A measurement of f.sub.CL in a signal processor is also possible. Accordingly, oscillator Coho is then detuned by an adjustment voltage ST which is proportional to f.sub.CL. This produces the representative spectrum at baseband shown in FIG. 2B.
Such a conventional method, which essentially operates with analog signals, and has the drawback that it is complicated and not cost-effective.